<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .clear {
            clear: both;
            background-color: olive;
        }
        .layout__main-wrapper,.layout__aside {
            float: left;
        }
        .layout__main-wrapper {
            width: 100%;
        }
        .layout__main {
            margin: 0 210px;
            background-color: mediumslateblue;
        }
        .layout__aside {
            width: 200px;
        }
        .layout__aside--left {
            margin-left: -100%;
            background-color: sandybrown;
        }
        .layout__aside--right {
            margin-left: -200px;
            background-color: springgreen;
        }
    </style>
</head>
<body>
        <div class="layout__main-wrapper">
            <div class="layout__main">主内容栏宽度自适应</div>
        </div>
        <aside class="layout__aside layout__aside--left">左侧边栏宽度固定</aside>
        <aside class="layout__aside layout__aside--right">右侧边栏宽度固定</aside>
        <footer class="clear">底部</footer>
</body>
</html>